Method of and apparatus for burning fuel



p 24, 1957 J. M. MILLER ETAL ma'm'on OF AND APPARATUS FOR BURNING FUELFiled Feb. 25. 1953 2 Sheets-Sheet 1 VII IIIIIIIA ll Ill IN VEN TORSla/w M. M/um W/u MM M. ape/mp1- ypo/V440 M. MCZfl/PEA/ Ai /DRIVE) l 4,1957 J. M. MILLER ETAL 2,807,223

METHOD OF AND APPARATUS FOR BURNING FUEL INVENTORS" (Jo/0v M- M/AAEAMammMl/RQH DoumaMA/clnfi'av ATTORNEY v Patented Sept. 24, 1957 1 METHODF AND APPARATUS FOR BURNENG FUEL John Matthewson Miller, Brookfield, andWilliam Mac- Donald Urquhart and Donald M. McLaren, Glasgow, Scotland,assignors to The Bahcock & Wilcox Company, New York, N. Y., acorporation of New Jersey Application February 25, 1953, Serial No.338,312

Claims priority, application Great Britain February 27, 1952 4 Claims.(Cl. 110-4055) The present invention relates to methods of and apparatusfor the combustion of solid fuels, and more particularly to theconstruction and operation of spreader stokers wherein fuel is projectedinto the furnace by a jet or jets of elastic fluid.

In accordance with the invention granular solid fuel the initialacceleration of the fuel occurring in a generally horizontal directionfollowed by an upward deflection to obtain a desirable distribution ofthe fuel transversely of the furnace. Advantageously, the elastic fluidmay consist of air at relatively low pressure and volume, with the airused for mixing and accelerated combustion of the fuel within thefurnace. The fines in granular solid fuel burn in suspension, while thecoarser particles are, in general, uniformly distributed over thetransverse area of the furnace. A moving grate is positioned in thelower portion of the furnace to support the ash and coarse particles offuel projected into the furnace by the spreader stoker. The fuel and ashsupporting surface of the moving grate moves from the front wall of thefurnace, beneath the position of the spreader stoker, toward theopposite wall.

The various features of novelty which characterize our invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which we have illustrated and described preferred embodimentsof our invention.

Of the drawings:

Fig. 1 is an elevation, in longitudinal section, of the lower portion ofa spreader stoker fired furnace constructed and arranged in accordancewith the invention;

Fig. 2 is an enlarged view of a portion of the apparatus shown in Fig.1;

, Fig. 3 is a plan view taken on line 33 of Fig. 1;

Fig. 4 is an enlarged view of another portion of the apparatus shown inFig. 1; and

Figs. 5 and 6 illustrate modified constructions of the spreader stokershown in Figs. 1 to 3, inclusive.

Referring to Figs. 1 to 4 of the drawings, the furnace has a front wall1 and a rear wall 2 at the ends of a combustion space 3 above atravelling grate 4 of the chain grate type arranged to that its upperrun moves rearwardly.

In front of and extending laterally of the front wall 1 is arranged ahopper 5, into which coal is led from an overhead bunker 6 through atraversing chute 7. At the bottom of the hopper and arranged side byside in a row extending laterally of the furnace are a number of feeders8 operating on the endless conveying surface principle. The feeders arearranged during operation to remove coal rearwardly from the hopper.Suitably all the feeders are driven'at the same speed by common drivingmeans. The amounts of'coal fed by the feeders can be adjustedindividually by raising or lowering guillois projected into a furnace bya jet of elastic fluid, with tine doors 9 positioned above the rear endsof the feeders and slidable within the rear wall 10, which is a doublewall of the hopper.

Coal is prevented from falling in front of the feeders by plates 11 andfrom falling between the feeders by ridged structures 12.

The coal from the rear ends of the feeders is arranged to fall onto acommon chute 13 sloping downwardly and rearwardly. At the bottom of thechute behind each feeder is a plate 14 of a respective spreader unit 15,which is arranged to receive the coal falling from the rear end of thefeeder. The chute is provided with structures 16 of refractory betweenadjacent spreader units, which are formed with sloping surfaces 17normal to the main surface of the chute and adapted to direct coalfalling thereon towards the nearmost spreader unit. The side surfaces 18of the structures 16 act to confine the falling coal to paths leadingthem to the spreader units.

Adjacent each spreader unit plate 14 the lowermost part of the chute isformed by a metal block 19. The surface of the chute above these blocks,between the refractory structures 16 and above the level thereof, isformed by the surfaces of refractory bricks 20.

A fixed scraper plate 21 is provided adjacent the rear of each feeder toinsure the falling from the feeder conveying surface of coal that mightotherwise adhere thereto. A screen 22 projecting downwardly andforwardly from the furnace front wall 1 near the bottom thereof isprovided for intercepting radiation from the furnace that might damagemechanical parts of the feeders.

In the illustrated embodiment of the invention, the center lines ofadjacent spreader units are 1' 9 /2" apart. Each spreader unit plate 14is bolted to the block 19 and has a shelf 23 extending horizontally andrearwardly from the bottom of the chute a distance of for example 5".Rearwardly of the shelf 23 is a deflector plate 24 providing a guidesurface which curves upwardly, the curvature being cylindrical around alaterally extending axis distant 6" from the upper surface of the plate.The plate 24 terminates where the slope thereof has an angle of 33 orthereabouts to the horizontal. The side edges 25 of the plate divergerearwardly, being 10% apart at the front edge of the shelf and 16" apartat the rear edge of the deflector plate 24. The side edges of the plateare furnished with upstanding guide walls 26 having their upper edgeshorizontal and at a height of 2 /2" above the level of the shelf 23 andprovided with forward and upward extensions 27 defining the path of thecoal falling over the part of the chute provided by the block '19. Thespreader unit is located so that the shelf 23 is 12 /8" above the grate.

The spreader stoker unit also comprises a horizontally directedelongated orifice 30 formed in the block 19. The orifice is 10" wide anddeep with its lower edge /2" above the plane of the shelf 23. Theorifice registers with a slot 31 in a duct 32 extending adjacent thelower ends of the chute transversely of the furnace and arranged tosupply air to all the spreader units.

The duct 32 is utilized to support the spreader units; to this end theblocks 19 are secured to the duct 32 by screws. The chute 13 is locatedwith respect to the duct 32 by brackets 33 bolted to the plates 34supporting the refractory blocks 20 and having curved parts conformingto the shape of the duct 32.

The space below much of the upper run of the grate is divided intocompartments 35-40 to which undergrate air is supplied under the controlof respective dampers 35a-40a. The compartments extend as zonestransversely of the grate and control of the respective quantities ofair thereto permits the rates of combustion attained at different zonesto be varied to suit the character of the coal to be burned.

At the rear end of the furnace the contents of the grate are led beneathan arch 45 at the bottom of the furnace rear wall 2, before beingremoved from the grate by the usual ash bars 46 at the rear end of thegrate. A vertical refractory faced shield 47 is provided in front of thearch 45, which shield has its front surface in the same plane as that ofthe rear Wall 2 above the arch and acts as a downward extension thereof,and at the lower end of the shield is a forwardly and upwardly inclinedledge 55. The ledge is welded to a pipe 54 whose center line is 1' 1 /2above the level of the main surface of the grate, and which is piercedjust above the level of the upper surface of the ledge to form anumberof orifices 56, each deep, side by side across the width of thefurnace adapted for the blowing from the ledge, forwardly into thefurnace, of coal falling onto the ledge. The pipe 54 is supplied withair suitably from the same source and under the same pressure as thatfor the duct 32.

The shield 47 is secured directly to, and the pipe 54 is supportedthrough supporting bars 57 from, supporting blocks 52 clamped at therear arch 45 between adjacent watertubes 53 of the rear wall 2. Theshield 47, the ledge 55, and the pipe 54 are suitably sectionali'zed,the sections being arranged closely side by side across the Width of thefurnace.

The furnace is adapted for the production of hot gases for steamgeneration. Appropriately the gases are led across heat absorbingsurfaces (not shown) comprising steam generating tubes and steamsuperheating tubes and subsequently through an economizer. Before thegases reach the economizer they may be passed through a classifier (notshown) for the removal of some of the grits in the gases, and thesegrits, comprising those having a substantial proportion of their weightin the form of combustible material, are added at 60 to the coalentering the traversing chute 7.

In the embodiment of the invention illustrated, the furnace isapproximately 14' x 7 6" in horizontal section, and three spreaderstoker units are installed in the front wall of the furnace. Whenburning 3500 to 4000 pounds per hour of coal having 10,000 to 12,000 B.t. u. per pound less than percent of the theoretical combustion air isintroduced through the orifices 30, with the remaining combustion airpassed upwardly through the grate 4.

In operation, coal prepared for spreader stoker firing, e. g. crushedand capable of passing through a /2" ring, is fed from the bunker 6through the traversing chute 7 to the hopper 5, and the feeders 8 areoperated to discharge coal continuously from the hopper to the spreaderunits 15. Air is supplied to the duct 32 at a pressure of 12" to 18"water gauge, and after issuing from the orifices 30 continuouslyprojects coal rearwardly from the spreader units. The coal particles orlumps so supplied to the furnace burn partly in suspension in thecombustion space above the fuel bed and partly in the fuel bed on thegrate 4, which is supplied with undergrate air from the compartments35-40. The ash arising from the combustion of the coal on the grate isdischarged at the rear of the grate. Air is also supplied to the pipe 54and reprojects forwardly into the furnace coal particles or lumps whichfind their way to the ledge 55.

The feeders 8 supply coal continuously to the spreader units 15. Infalling downwardly over the chute in contact with or near the refractorybricks 20, the coal particles or lumps are subjected to drying effectsby the radiation they receive from the furnace combustion space 3 andthe heat they receive by contact or radiation from the radi-antly heatedrefractory bricks 20.

The spreader unit plates 14 at the bottom of the ramp 13 arrest thedownward progress of the coal. When the coal particles or lumps areacted upon by the currents of air from the orifices 30, they undergorearward accelerations. During such accelerations the shelf 23 preventsthe coal as a whole from falling. The shelf surfaces are generallyhorizontal, and the acceleration of a particle or lump while it is on ashelf is larger than if the shelf were directed rearwardly and upwardly.The deflector 24 at the end of the shelf 23 deflects the relatively fastmoving lump or particle so that it leaves the spreader unit in a pathhaving an upward inclination to the horizontal, the angle of which Willgenerally be relatively large. By reason of the large angles to thehorizontal and the speed at which the coal lumps or particles leave thespreader units, the coal particles or lumps destined to burn out on thegrate tend to have trajectories which carry them large distancesrearwardly before they reach the fuel bed on the grate. It is foundthatin this way coal lumps or particles can be distributed over an areawhich is relatively great from front to rear, so that the furnace canhave a deep combustion space. The currents of air from the orifices 30fan out to a certain extent laterally, while moreover adjacent coalparticles or lumps on each spreader unit plate 14 tend to be blownapart. Coal lumps and particles are, therefore, distributed also to eachside of each spreader unit at the rear thereof, so that a satisfactoryfuel bed is formed on the grate which extends across the whole widththereof.

The air from the orifices 30 and those coal particles or lumps designedto fall on the fuel bed on the grate travel at least initially in thesame general direction with the air sweeping past such lumps orparticles. The lumps or particles are therefore scrubbed by the air sothat ample oxygen for rapid ignition and combustion of such particles orlumps is available.

The larger lumps of coal, inasmuch as they have a smaller ratio ofsurface area to weight, tend to be less greatly accelerated by the airfrom the orifices 30 and less sustained by the mass of gases risinggenerally upwardly from the fuel bed on the grate, and consequently theytend to fall in greater proportion towards the front end of the fuel bedon the grate. Larger lumps require a longer time for their combustionthan smaller lumps or particles, and this longer time they receivebecause the upper run of the grate moves rearwardly. The lumps orparticles falling near the rear of the fuel bed on the grate tend to besuch as are more greatly accelerated by the air currents from theorifices and more greatly sustained by the gas mass rising generallyupwardly from the fuel bed, i. e. they are generally the smaller lumpsor particles; consequently although a less time is available to them forcombustion in the fuel bed before they are discharged from the grate,they will be generally burnt out on the grate before they reach the ashbars 46, since they require only a shorter time to burn.

The very fine particles tend not to fall onto the fuel bed on the grate,but to be carried upwardly in the space above the fuel bed. Suchparticles tend to follow in a particularly high degree the air from theorifices 30 as it enters the mass of gases rising upwardly from the fuelbed. Much of the air penetrates horizontally or nearly hori- Zontallyinto the said upwardly rising gas mass, while other parts of the aireddy above the spreader units, as shown at 70, being drawn again towardsthe original direction of motion from the orifices 30. The exit of thevery fine particles from the furnace is thus delayed so that theirburning out in suspension before they leave the combustion space ispromoted.

Shield 47 presents to the combustion space a surface from which coalrebounds onto the grate in a forward direction, or which leads to theledge 55 many coal lumps or particles falling from the rear wall 2 andfrom the shield itself. From the ledge 55 air discharging forwardlythrough the orifices 56 projects the coal lumps or particles forwardlyinto the furnace. The coal lumps or particles which impinge onto theshield at the end of their trajectories from the spreader units mightnot, if the shield were absent and they fell on the grate, have time toburn out before being carried by the grate to the ash bars 46, but byvirtue of their re-delivery in a forward direction so that they fallonto the fuel bed on the grate in front of the plane of the rear Wall 2sufiicient time is given to such lumps or particles to make possible thecompletion of their combustion in the furnace. Moreover, the shield 47reduces the possibility of particles or lumps in the rain of coalprojected rea'rwardly from the spreader units pursuing trajectorieswhich would carry them onto or beyond the ash bars 46; at the same timethe shield 47 constrains air flowing upwardly through the grate belowthe rear arch 45 to flow in a forward direction above the grate andthrough the relatively narrow opening between the pipe 54 and the fuelbed on the grate, and this movement of air and gases is adapted toshorten the trajectories of lumps or particles which might otherwisefall too near the end of the grate.

We have found that with the apparatus described most grades of coaladapted for spreader stoker firing may be satisfactorily spread ontogrates for carrying fuel beds having lengths of from 14 to 20 feet.

The amount of air delivered to the furnace through the orifices 56 isonly a small fraction of the total air supplied. The amount of airsupplied by the orifices 30 is greater, but to provide it requires aminimum of auxiliary power, for although the air is admitted through theorifices 30 with a velocity suflicient to achieve a satisfactorydistribution of fuel by the spreader units 15 over the grate, theaggregate area of the orifices 30 is limited. It is found that the airquantity is substantially less than the normal requirements for overfireair, so that no inefficiency arises by the presence of an excessiveproportion of excess air in the gases leaving the furnace. Overfire airthat may be required in addition to that from the orifices 30 isprovided through suitable secondary air nozzles. The high velocity ofthe air from the orifices 30 is a factor causing an advantageous degreeof turbulence in the gas mass rising from the fuel bed, and thuspromoting complete combustion. The pressure of air required in the duct32 is not however greater than that readily obtainable from a singlestage fan.

The turbulence above the fuel bed and the capability of admitting moreair to some zonw of the grate than to others are features rendering thefurnace particularly adapted for efiiciently burning high ash coals.

When the fresh coal supplied is wet, the addition thereto at 60 of thegrits collected from the gases by the classifier above referred to, and.to be refired, increases the ease with which the coal is distributedover the grate, no doubt by reducing, since the grits are very dry, thetendency of wet fine coal particles to adhere to one another.

For the rest, the advantages of e. g. unrestricted ignition of the coal,flexibility in following load fluctuations, little tendency to formlarge clinkers, facility of burning highly swelling coals, and otheradvantages associated with spreader stoker firing are achieved in thefurnace de scribed.

It is found that, as compared with spreader stoker firing by mechanicalcoal sprinkler means, fewer grits are carried over from the furnace,while the proportion of combustible material in the grits carried overis smaller.

The means at the lower end of the furnace rear wall 2 arranged forre-directing coal particles forwardly within the furnace are disclosedand claimed in a copending application Serial No. 338,797, filedFebruary 25, 1953, in the name of William M. Urquhart.

Fig. 5 is a sectional view similar to Fig. 2 but illustrating amodification of the spreader unit in which the deflector 24 instead ofbeing curved and integral with the shelf 23 comprises a flat platewelded to the rear end of the shelf 23 and extending upwardly andrearwardly at an angle of 33 or thereabouts.

Fig. 6 is another sectional view similar to Fig. 2, illustrating amodification in which the lower lip of the orifice 30 is in the sameplane as the upper surface of the shelf 23. It is found appropriate touse this construction should 6 the coal have too great a tendency, byreason of undue wetness, to adhere to the shelf 23.

While in accordance with the provisions of the statutes we haveillustrated and described herein the best form of the invention nowknown to us, those skilled in the art will understand that changes maybemade in the form of the apparatus disclosed without departing from thespirit of the invention covered by our claims, and that certain featuresof our invention may sometimes be used to advantage without acorresponding use of other features.

What is claimed is:

1. The method of operating a spreader stoker fired furnace whichcomprises continuously gravitating crushed solid fuel downwardlyadjacent an inner wall face of said furnace to a position intermediatethe furnace height, initially accelerating said solid fuel in asubstantially horizontal direction by a jet of air and thereafterdeflecting said fuel and jet of air in an upwardly inclined directionacross said furnace, collecting coarse fuel and ash in the lower portionof said furnace, and continuously moving said coarse fuel and ash in agenerally horizontal direction across said furnace from a positionadjacent the fuel inlet, and in a direction generally along the axis ofthe jet, whereby the length of the path of travel of fuel on the fuelbed through the furnace is in a substantially direct proportion to theparticle size.

2. The method of operating a spreader stoker fired furnace whichcomprises continuously gravitating crushed solid fuel downwardlyadjacent an inner wall face of said furnace to a position intermediatethe furnace height, initially accelerating said solid fuel in asubstantially horizontal direction by a fluid jet and thereafterdeflecting said fuel and fluid jet in an upwardly inclined directionacross said furnace, collecting coarse fuel and ash in the lower portionof said furnace, and continuously moving said coarse fuel and ash acrosssaid furnace and in a direction generally parallel to the axis of thejet, whereby the length of the path of travel of fuel on the fuel bedthrough the furnace is in a substantially direct proportion to theparticle size.

3. Combustion apparatus comprising walls defining a furnace, ahorizontally disposed shelf positioned along the wall of said furnaceintermediate the height thereof, an upwardly inclined deflector mergingwith said shelf at a position spaced from said wall inwardly of saidfurnace, means for continuously delivering solid fuel at a selected rateto said shelf, air jet means for projecting said fuel across said shelfand deflector into said furnace, and a moving grate positioned in thelower portion of said furnace to collect ash and solid material thereon,said grate adapted to move in a horizontal direction generally along theaxis of the jet whereby the length of the path of travel of fuel throughsaid furnace on said grate is in a substantially direct proportion tothe particle size.

4. Combustion apparatus comprising walls defining a furnace, ahorizontally disposed shelf positioned along one wall of said furnaceintermediate the height thereof, an upwardly inclined deflector mergingwith said shelf at a position spaced from said wall inwardly of saidfurnace, means for continuously delivering solid fuel at a selected rateto said shelf, means for discharging an elastic fluid jet horizontallyacross the upper surface of said shelf, and a moving grate positioned inthe lower portion of said furnace to collect ash and solid materialthereon, said grate adapted to move in a horizontal direction generallyalong the axis of the jet whereby the length of the path of travel offuel through said furnace on said grate is in a substantially directproportion to the particle size.

References Cited in the file of this patent UNITED STATES PATENTS803,805 Brewster Nov. 7, 1905 979,850 Hanna Dec. 27, 1910 (Otherreferences on following page) 7 Shults Sept. 1, 1914 Jackson et a1. Apr.15, 1919 Kraemer Mar. 31, 1925 Kohout Jan. 8, 1929 Crow et a1 June 18,1929 Kohout Oct. 31, 1939 Brown Jan. 23, 1940 & Clausen et a1 Feb. 19,1946 Glaeser Oct. 4, 1949 FOREIGN PATENTS Great Britain July 1, 1935France Sept. 10, 1934 Grmany July 3, 1937

